1. Field of the Invention
This invention relates to a toner used in recording processes utilizing electrophotography, electrostatic recording, magnetic recording or toner-jet recording. More particularly, this invention relates to a toner used in copying machines, printers and facsimile machines in which a toner image is formed previously on an electrostatic latent image bearing member and thereafter the toner image is transferred to a transfer medium to form an image, and also relates to a two-component developer, an image forming method and an apparatus unit which make use of the toner.
2. Related Background Art
Image forming apparatus are well known conventionally in which an electrostatic latent image is formed on a photosensitive member (drum) by means of an exposure optical system, the electrostatic latent image formed is developed by a developing apparatus to form a toner image and the toner image formed is transferred to recording paper and then fixed thereto.
Developers used in such a developing apparatus include a one-component developer and a two-component developer. In the one-component developer, toner particles are charged electrostatically by friction between toner particles one another or friction with a suitable charging member, and the toner particles thus charged are carried by a developing sleeve of the developing apparatus and then come to adhere to latent image areas on the surface of the photosensitive member to form a toner image.
Now, in the formation of such a toner image, especially in the case of the one-component developer, a lowering of fluidity of the developer because of, e.g., leaving a developing assembly to stand for a long period of time may result in a strong adhesion between toner particles to make it impossible to effect satisfactory charging of the toner particles, so that what is called "uneven images" or "dimmed images" occurs, which is a phenomenon such that visible images are formed non-uniformly even though latent images are uniform. As a method for preventing it, conventionally put into wide use is a method of agitating the developer previously in the developing apparatus to impart fluidity thereto.
However, any excessive agitation of the developer may accelerate toner deterioration, which has been a cause of short service life of developers.
The two-component developer is constituted of magnetic carrier particles and non-magnetic toner particles made of a synthetic resin, blended in an appropriate blend ratio. The toner particles are charged electrostatically upon mixing with the carrier particles, and the toner particles thus charged are carried by a developing sleeve of the developing apparatus and then come to adhere to latent image areas on the surface of the photosensitive member to form a toner image. As a developing method making use of such a two-component developer, what is called magnetic-brush development is disclosed in, e.g., Japanese Patent Applications Laid-Open No. 55-32060 and No. 59-165082, in which a magnetic brush is formed on the surface of a developing sleeve provided internally with a magnet, by the use of a two-component developer comprised of carrier particles and toner particles, the magnetic brush thus formed is rubbed against, or brought close to, a photosensitive drum opposed to the developing sleeve while keeping a minute development gap between them, and an alternating electric field is applied continuously across the developing sleeve and the photosensitive drum (between S-D) to cause the toner particles repeatedly to transit from the developing sleeve side to the photosensitive drum side and vice versa, to carry out development.
In such magnetic brush development making use of a two-component developer, the toner particles are charged triboelectrically by mixing them with carrier particles. Since the carrier particles have a higher specific gravity than the toner particles, the toner particles undergo a high mechanical strain because of their friction with the carrier particles when mixed, so that the deterioration of toner tends to accelerate with the progress of development operated repeatedly.
Once such deterioration of toner has occurred, it may cause concretely such phenomena that the density of fixed images changes as a result of long-term service, that the toner particles adhere partly to non-image areas to cause what is called "fog" and that minute-image reproducibility becomes poor.
As a result of extensive studies, the present inventors have elucidated that the above toner deterioration has relation to the following three phenomena.
The first phenomenon is break of toner particles into fine particles.
When toners whose particles have a rugged shape and are individually different in shape, as typified by pulverization toners commonly used, are agitated in the developing apparatus over a long period of time, it has been revealed that the toner particles break especially at their convexes to become fine particles as a result of collision of the toner particles against a developer carrying member or against toner particles one another.
The second phenomenon is that particles of an external additive become buried in toner particle surfaces ("surfaces" used in this context are herein meant to be outermost layer portions).
When the toners whose particles have a rugged shape and are individually different in shape as in pulverization toners are used, fine particles used as external additive particles stand buried in the surfaces of toner particles at their convexes, whereas the external additive particles have been found not to be buried at their concaves. Meanwhile, when toner particles having spherical particle shapes as typified by polymerization toners are used, it has been revealed that the toner particles neither break nor become fine particles but fine particles added as an external additive stand buried uniformly in the surfaces of toner particles.
The third phenomenon is that toner particles become non-uniform in charging performance.
In use of conventionally known commonly available toner particles, measurement of their charge distribution has revealed that the charge distribution becomes broad when toner particles are agitated in the developing apparatus over a long period of time, compared with that before agitation.